1. Field of the Invention
The invention relates to a digital input unit for an automation device having at least one current-drawing DC input channel for connecting a transducer operated at a nominal DC voltage, where the input channel is configured to set the current based on the input voltage according to an input characteristic curve.
2. Description of the Related Art
Such a digital input unit is known from the Siemens device handbook “SIMATIC, S7-300, Automatisierungssystem S7-300, Baugruppendaten” [SIMATIC, S7-300, S7-300 automation system, assembly data], chapter 3, issue 08/2009. A transducer can be connected to one of the input channels, for example, a transducer in the form of a Bero proximity switch which is designed for a nominal DC voltage of 24 or 48 V, an input characteristic curve complying with a so-called type 1 or type 2 characteristic according to the IEC 61131 or EN 61131 standard.
This standard prescribes current and voltage limits for 0 and 1 signal states for the type 1 and type 2 characteristics, the current limits according to the type 2 characteristic being higher than those predefined according to the type 1 characteristic. According to the type 1 characteristic, the upper current limit for a 0 or 1 signal state is 15 mA, but is 30 mA according to the type 2 characteristic. In other words: a type 2 digital input unit has a more current-intensive characteristic than a type 1 digital input unit.
The situation may then occur in which, on account of a customer requirement, a digital input unit must perform functions for a multiplicity of transducers of different nominal DC voltages. In this case, the digital input unit must be designed for a multiplicity of DC input voltages, for example in a range from 24 V to 125 V. Such a “wide-range input” of 24 V to 125 V results in a high power loss and thermal load in the case of DC input voltages of greater than the 30 V voltage mentioned with regard to the 1 signal state on account of the current-intensive type 2 characteristic for this 1 signal state, which has a disadvantageous effect on the service life of the digital input unit.